Reduced bone formation and osteoporosis are serious complications of type I diabetes (T1D) that predispose diabetic patients to increased fracture risk and reduced quality of life. Therefore, developing therapeutic strategies to target T1D bone loss is a critical need for diabetic patients. We have identified that oral administration of the probiotic L. reuteri strikingly prevents T1D femur and vertebral bone loss in two distinct T1D models of mice. Because probiotics, ingested bacteria that benefit health, modulate the gastrointestinal (GI) system, their beneficial effect on bone formation is intriguing and significat. Recent studies recognize the relationship between the GI system and bone as a critical factor in the regulation of bone density. In fact, intestinal inflammation and the gut microbiota have both been acknowledged as crucial players in the physiology of gut-bone axis. Given their beneficial effect on the GI system, probiotics are increasingly used to treat diseases such as colic and inflammatory bowel disease (IBD). The long-term goal of our research is to test the potential use of a probiotic therapeutic for treating T1D bone loss in human patients. The objective of this proposal is to further understand the mechanistic basis by which L. reuteri modifies gut-bone axis and prevents bone loss. Our data indicate that LR has beneficial effects in the gut including decreased inflammation, enhancement of barrier function, and microbial community composition changes. These changes appear to translate into improved bone health in mice. Our data further demonstrate that L. reuteri reverses T1D-suppression of skeletal Wnt10b, a critical enhancer of osteoblast lineage, differentiation and bone formation. Interestingly, Wnt10b expression and osteoblast viability are critically regulated by bone marrow-derived TNF? in T1D. Together our pilot data implicates gut microenvironment and bone TNF?-Wnt10b axis as potential mechanistic targets of LR for preventing T1D bone loss. Based on our novel findings we hypothesize that LR treatment prevents T1D bone loss through its actions on the gut and/or bone microenvironment by preventing intestinal inflammation and osteoblast death. We will test this hypothesis by 1) Identifying the mechanistic basis by which LR regulates the gut environment to prevent bone loss in T1D male mice and 2) Determining the host mechanisms by which LR treatment modulates bone microenvironment and prevents T1D-induced bone inflammation and bone loss. The bone anabolic properties of L. reuteri and its impeccable safety record make it a novel bio- therapeutic option with minimal side effects to reduce osteoporosis and fracture risk in T1D patients. Our studies could provide a paradigm shift in understanding the role of gut-bone signaling axis in T1D.